Passivation of Nonlinear Optical Crystals

ABSTRACT

A laser system includes a nonlinear optical (NLO) crystal, wherein the NLO crystal is annealed within a selected temperature range. The NLO crystal is passivated with at least one of hydrogen, deuterium, a hydrogen-containing compound or a deuterium-containing compound to a selected passivation level. The system further includes at least one light source, wherein at least one light source is configured to generate light of a selected wavelength and at least one light source is configured to transmit light through the NLO crystal. The system further includes a crystal housing unit configured to house the NLO crystal.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims the benefit of theearliest available effective filing date(s) from the following listedapplications: The present application constitutes a continuation patentapplication of U.S. patent application Ser. No. 15/284,231, filed onOct. 3, 2016, which in turn constitutes a continuation patentapplication of U.S. patent application Ser. No. 15/010,331, filed onJan. 29, 2016, which in turn constitutes a continuation patentapplication of U.S. patent application Ser. No. 13/488,635, filed onJun. 5, 2012, which in turn constitutes a regular (non-provisional)patent application of U.S. Provisional Patent Application No.61/544,425, filed on Oct. 7, 2011, whereby each of the above-listedpatent applications is incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to the field of nonlinear opticalmaterials, and in particular to a system and method for passivatingnonlinear optical crystals to cure crystal defects.

BACKGROUND

Many modern-day laser systems require nonlinear optical (NLO) elements.For example, NLO elements are commonly used in applications such asfrequency mixing (e.g. harmonic generation, parametricgeneration/amplification, and the like), Raman amplification, Kerr-lensmode-locking, electro-optic modulation, acousto-optic modulation, andothers.

Laser-induced damage (LID) of NLO elements is a major limitation of manymodern laser systems. LID occurs as a result of the interaction betweenlaser radiation and the material making up a given NLO element.Accordingly, over time, NLO elements incur LID, which may negativelyimpact such physical properties as transmittance, reflectivity,refraction indices, and the like. In turn, this degradation of physicalproperties due to accrued LID eventually leads to failure of NLOelements within a laser system.

LID becomes even more problematic in laser systems that utilize shorterwavelengths of the electromagnetic spectrum, such as deep ultraviolet(DUV) light, with wavelengths less than 300 nm. In addition,laser-induced damage rates are also impacted by material defects presentin NLO elements, such as dislocations, impurities, vacancies, and thelike. In most cases, material defects in a given NLO element leads tothe NLO element being less resistant to LID. Accordingly, the NLOelements have a shorter lifetime as a result of material defects.

The present invention is directed to mitigating the foregoing problemsby improving damage resistance of NLO elements utilizing a novel systemand method disclosed herein.

SUMMARY

A laser system is disclosed, in accordance with one or more embodimentsof the present disclosure. In one embodiment, the laser system includesa nonlinear optical (NLO) crystal. In another embodiment, the NLOcrystal is annealed within a selected temperature range. In anotherembodiment, the NLO crystal is passivated with at least one of hydrogen,deuterium, a hydrogen-containing compound or a deuterium-containingcompound to a selected passivation level. In another embodiment, thelaser system includes at least one light source. In another embodiment,the laser system is configured to generate light of a selectedwavelength. In another embodiment, the light source is furtherconfigured to transmit light through the NLO crystal. In anotherembodiment, the laser system includes a crystal housing unit configuredto house the NLO crystal.

A nonlinear optical (NLO) crystal is disclosed, in accordance with oneor more embodiments of the present disclosure. In one embodiment, theNLO crystal is annealed within a selected temperature range. In anotherembodiment, the NLO crystal is passivated with at least one of hydrogen,deuterium, a hydrogen-containing compound or a deuterium-containingcompound to a selected passivation level.

A method for passivating crystal defects of a nonlinear (NLO) crystal isdisclosed, in accordance with one or more embodiments of the presentdisclosure. In one embodiment, the method includes providing a nonlinearoptical (NLO) crystal. In another embodiment, the method includesmaintaining a temperature of the NLO crystal within a selectedtemperature range below a melting temperature of the NLO crystal. Inanother embodiment, the method includes exposing the NLO crystal topassivating gas having a concentration of at least one of hydrogen,deuterium, a hydrogen-containing compound and a deuterium-containingcompound at or near a selected concentration to repair at least one ofdangling bonds or broken bonds within the NLO crystal.

A method for passivating crystal defects of a nonlinear (NLO) crystal isdisclosed, in accordance with one or more embodiments of the presentdisclosure. In one embodiment, the method includes providing an NLOcrystal. In another embodiment, the method includes performing anannealing process on the NLO crystal to reduce water or OH content ofthe NLO crystal. In another embodiment, the method includes exposing theNLO crystal to passivating gas having a concentration of at least one ofhydrogen, deuterium, a hydrogen-containing compound and adeuterium-containing compound at or near a selected concentration torepair at least one of dangling bonds or broken bonds within the NLOcrystal.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1A is a block diagram illustrating a system for passivating a NLOcrystal, in accordance with one embodiment of the present invention.

FIG. 1B illustrates a conceptual view of an exposure chamber of a systemfor passivating a NLO crystal, in accordance with one embodiment of thepresent invention.

FIG. 2A is a flow diagram illustrating a method for passivating a NLOcrystal, in accordance with one embodiment of the present invention.

FIG. 2B is a flow diagram illustrating a method for passivating a NLOcrystal, in accordance with one embodiment of the present invention.

FIG. 2C is a flow diagram illustrating a method for passivating a NLOcrystal, in accordance with one embodiment of the present invention.

FIG. 2D is a flow diagram illustrating a method for passivating a NLOcrystal, in accordance with one embodiment of the present invention.

FIG. 3A is a flow diagram illustrating a method for annealing andpassivating a NLO crystal, in accordance with one embodiment of thepresent invention.

FIG. 3B is a flow diagram illustrating a method for annealing andpassivating a NLO crystal, in accordance with one embodiment of thepresent invention.

FIG. 3C is a flow diagram illustrating a method for annealing andpassivating a NLO crystal, in accordance with one embodiment of thepresent invention.

FIG. 3D is a flow diagram illustrating a method for annealing andpassivating a NLO crystal, in accordance with one embodiment of thepresent invention.

FIG. 4 is a block diagram illustrating a laser system equipped with anannealed and passivated NLO crystal, in accordance with one embodimentof the present invention.

FIG. 5 is a block diagram illustrating a system for inspecting a waferor a photomask, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Referring generally to FIGS. 1A through 5, a system and method forpassivating a nonlinear optical (NLO) crystal is described in accordancewith the present disclosure. Laser systems commonly utilize NLO crystalsfor many applications such as frequency mixing, Raman amplification,Kerr-lens mode-locking, electro-optic modulation, and acousto-opticmodulation, among others. Exposure to electromagnetic radiation within alaser system affects physical properties (e.g., transmittance,reflectivity, refraction indices, etc.) of NLO crystals. The resultingchanges to the physical properties of NLO crystals are commonly referredto as laser-induced damage (LID) and tend to impair NLO crystals fromfunctioning properly. NLO crystals are less resistant to LID when theyhave a greater quantity or amount of crystal defects such asdislocations, impurities, vacancies, and the like. The present inventionis directed to a system and method for curing crystal defects of an NLOcrystal utilizing hydrogen passivation and/or crystal annealing.

As used throughout the present disclosure, the term “crystal”, “NLOcrystal”, or “nonlinear crystal” generally refers to a nonlinear opticalcrystal suitable for frequency conversion. For example, the nonlinearoptical crystal of the present invention may be configured to frequencyconvert incident illumination of a first wavelength (e.g., 532 nm) to anoutput illumination of a shorter wavelength (e.g., 266 nm). Further, thenonlinear optical crystal of the present invention may include, but isnot limited to, beta-Barium Borate (BBO), Lithium Triborate (LBO),Lithium Tetraborate (LTB), Cesium Lithium Borate (CLBO), Cesium Borate(CBO), oxide-type non-linear crystals, and the like.

As used throughout the present disclosure, the term “wafer” generallyrefers to a substrate formed of a semiconductor or non-semiconductormaterial. For example, semiconductor or non-semiconductor materialsinclude, but are not limited to, monocrystalline silicon, galliumarsenide, and indium phosphide. A wafer may include one or more layers.For example, such layers may include, but are not limited to, a resist,a dielectric material, a conductive material, and a semiconductivematerial. Many different types of such layers are known in the art, andthe term wafer as used herein is intended to encompass a wafer on whichall types of such layers may be formed.

FIGS. 1A and 1B illustrate a system 100 for passivating a NLO crystal104 in order to cure crystal defects within the crystal. These defectsmay be cured through the attachment of hydrogen atoms to dangling orbroken bonds within the crystal 104. For example, the dangling or brokenbonds may include dangling oxygen bonds, which are often a primary typeof defect that affects physical/optical properties as well as NLOcrystal lifetime. In one embodiment, the system 100 may include anexposure chamber 101 configured to contain a volume of passivating gas.The exposure chamber 101 may be further configured to contain the NLOcrystal 104 such that the NLO crystal 104 may be exposed to thepassivating gas contained within the exposure chamber 101. In addition,the exposure chamber 101 may be further configured to contain asubstrate 102 configured to hold the NLO crystal 104 while the NLOcrystal 104 is exposed to passivating gas contained within the exposurechamber 101. Alternatively, the substrate 102 may be a portion of aninterior surface of the chamber 101.

The passivating gas of the present invention may include a gaseousmixture of two or more gases having a selected concentration ofhydrogen. In one embodiment, the gas mixture may include molecularhydrogen (H₂). In another embodiment, the passivating gas may include alow-molecular-weight gas that may yield hydrogen upon chemical reactionor dissociation. Such low-molecular-weight gases may include, but arenot limited to, NH₃ or CH₄. The desired concentration of hydrogen mayinclude a concentration exceeding the natural abundance of hydrogenpresent under normal atmospheric conditions. In this regard, thehydrogen concentration of the passivating gas may consist of aconcentration in excess of the hydrogen concentration naturally presentin air. In another aspect, the desired concentration of hydrogen mayalso be a user selected concentration or a concentration determinedutilizing one or more physical attributes of the NLO crystal 104. Thepassivating gas mixture may further include an inert gas such as argon,nitrogen, helium or the like.

In a further embodiment, the passivating gas of the present inventionmay include a gas mixture having a hydrogen concentration in the rangeof 5 to 10%. It is noted herein that this hydrogen concentration rangeis not a limitation and is presented merely for purposes ofillustration. It is contemplated that the hydrogen concentration levelof the passivating gas may include any range suitable for the givenapplication. In a further embodiment, the hydrogen concentration of thepassivating gas mixture may include a heavy isotope of hydrogen,deuterium, for improved passivation results. The exact amount ofdeuterium in the mixture may be determined by optimizing passivationresults and may vary from a fraction of total hydrogen concentration to100% of all the hydrogen in the mixture.

In an embodiment, the system may further include a passivating gassource 108 fluidically coupled to the exposure chamber 101 andconfigured to supply the exposure chamber with passivating gas. Theexposure chamber 101 may include a gas inflow port 105 configured toreceive passivating gas from the passivating gas source 108 and furtherconfigured to transmit passivating gas received from the passivating gassource 108 to an interior portion of the exposure chamber 101. Theexposure chamber 101 may further include a gas outflow port 106configured to release passivating gas from the interior portion of theexposure chamber 101.

In a further embodiment, the system 100 may include a flow controller110 fluidically connected in between the passivating gas source 108 andthe exposure chamber 101. The flow controller 110 may be configured tocontrol the rate at which passivating gas is supplied to the exposurechamber 101. The flow controller 110 may include a valve, regulator, orany other means for regulating the pressure or rate at which passivatinggas moves through at least one conduit fluidically connecting the flowcontroller 110 to the exposure chamber 101. The flow controller may befurther configured to be fluidically connected to the gas inflow port105 of the exposure chamber and further configured to control the rateat which passivating gas is supplied through the gas inflow port 105 tothe interior portion of the exposure chamber 101. In another embodiment,the flow controller 110 or an additional flow controller (not shown) maybe configured to be fluidically connected to the gas outflow port 106 ofthe exposure chamber 101 and further configured to control the rate atwhich passivating gas is removed from the interior portion of theexposure chamber 101.

In another embodiment, the system 100 may further include one or morecomputing systems 112 communicatively coupled to the flow controller110. The computing system 112 may be configured to provide the flowcontroller 110 with instructions for controlling the rate at whichpassivating gas is supplied to the exposure chamber 101. The computingsystem 112 may be further configured to provide the flow controller 110or an additional flow controller (not shown) with instructions forcontrolling the rate at which passivating gas is removed from theexposure chamber 101. The computing system may contain a carrier medium114 such as a flash, solid-state, optical, random access or other staticor dynamic memory device configured with program instructions 116including a flow control algorithm 118. Flow control algorithms 118 areknown to the art, such as algorithms for configuring a pressure valvethat may be included in the flow controller 110. For example, the flowcontrol algorithm 118 may direct the flow controller 110 to actuate thepressure valve based on a correlation between the pressure valve'smechanical properties and a desired flow rate. In some embodiments, auser selected flow rate of 10 to 200 cm³/min may be a desirable flowrate for passivating the NLO crystal 104 contained within the exposurechamber 101. However, flow rates outside of the 10 to 200 cm³/min rangemay be desirable depending on the passivating gas mixture or thecomposition of the NLO crystal 104. The foregoing flow rate range isexemplary only and is not intended to limit the present invention in anyway.

In a further embodiment, the substrate 102 configured to hold the NLOcrystal 104 within the exposure chamber 101 may be further configured tocontrol the temperature of the NLO crystal 104. In one aspect, a usermay select a temperature greater than ambient or room temperature, butless than the melting temperature of the NLO crystal 104. For example,the substrate 102 may be configured to heat the NLO crystal 104 to arange of 300 to 350° C. or some other selected temperature to improvehydrogen penetration into the crystal, alleviate decomposition ofmolecular hydrogen (e.g., H₂) or other hydrogen-containing molecule intoatomic hydrogen, or eliminate undesirable reaction products betweenhydrogen and the NLO crystal 104 (e.g. weak OH bonds, water, or thelike). It is contemplated herein that the substrate 102 may beconfigured to increase, decrease, and/or maintain the temperature of theNLO crystal 104 at any feasible temperature or range of temperaturesdesirable for successfully passivating the NLO crystal 104. Accordingly,the foregoing temperature range is exemplary only and is not intended tolimit the present invention in any way.

In accordance with the foregoing system 100, FIGS. 2A through 2Dillustrate flow diagrams for a method 200 for passivating the NLOcrystal 104 with hydrogen in order to cure crystal defects caused bydangling or broken bonds. Referring to FIG. 2A, the method 202 mayinclude one or more of the following steps: (i) step 202, maintainingthe temperature of the NLO crystal 104 at or near a selected temperaturethat is a user selected temperature or a temperature determinedutilizing one or more attributes of the NLO crystal 104 (e.g.composition, water content, defect level, etc.); and (ii) step 204,exposing the NLO crystal 104 to passivating gas having a selectedconcentration of hydrogen that is a user selected hydrogen concentrationor a hydrogen concentration determined utilizing one or more attributesof the NLO crystal 104.

In step 202, the temperature of the NLO crystal 104 may be controlled byany heating and/or cooling element (hereinafter “heating element”) suchas the substrate 102 configured to hold the NLO crystal 104 in theexposure chamber 101 of the system 100. The heating element may beconfigured to heat or cool the NLO crystal 104 to the selectedtemperature which may be a user selected temperature, a temperaturedetermined utilizing one or more attributes of the NLO crystal 104, orany temperature that improves hydrogen penetration into the crystal,alleviates decomposition of H₂ molecules into H atoms, or eliminatesundesirable products from one or more reactions between hydrogen and theNLO crystal 104 (e.g. weak OH bonds, water, etc.). For example, in oneembodiment the selected temperature may be a temperature in the range ofapproximately 300 to 350° C. The heating element may be furtherconfigured to maintain the temperature of the NLO crystal 104 at or nearthe selected temperature for a selected period of time such as the timerequired to adequately passivate the NLO crystal 104. For example, thetime required to adequately passivate the NLO crystal 104 may be in therange of approximately 100 to 200 hours. Accordingly, in one embodiment,the heating element may be configured to maintain the temperature of theNLO crystal 104 at or near the selected temperature for the selectedperiod of time in the range of approximately 100 to 200 hours. Theforegoing temperatures and time durations are included by way of exampleonly, and it is contemplated that these parameters may be significantlyaltered without departing from the essence of this disclosure.Accordingly, nothing herein should be construed to limit the presentinvention in any way.

In step 204, the NLO crystal 104 may be exposed to passivating gaswithin an atmospherically controlled container such as the exposurechamber 101 of the system 100. The passivating gas may be a gas mixturehaving a selected concentration of hydrogen. The selected hydrogenconcentration may be a user selected concentration, a concentrationdetermined utilizing one or more attributes of the NLO crystal 104, orany acceptable concentration for curing crystal defects of the NLOcrystal 104 by attaching hydrogen atoms from the passivating gas tobroken or dangling bonds of the NLO crystal 104. For example, in oneembodiment, the selected hydrogen concentration of the passivating gasmay be a hydrogen concentration in the range of approximately 5 to 10%of the passivating gas mixture. However, the foregoing hydrogenconcentration is only included by way of example, and it is not intendedto limit the present invention in any way.

Referring to FIG. 2B, step 204 may include a step 206 of maintaining theflow rate at which the passivating gas may flow through the container ator near a selected flow rate such as a user selected flow rate, a flowrate determined utilizing one or more attributes of the NLO crystal 104,a flow rate acceptable for maintaining the hydrogen concentration ofpassivating gas within the container at or near the selected hydrogenconcentration, or any flow rate sufficient for curing crystal defects ofthe NLO crystal 104 by attaching hydrogen atoms from the passivating gasto broken or dangling bonds of the NLO crystal 104. The flow rate may beregulated by the flow controller 110 of the system 100 or by any valve,regulator, or other means for controlling the pressure or rate at whichgas moves through one or more conduits. For example, in one embodiment,the flow controller 110 may be configured to regulate the flow rate ofpassivating gas flowing through the exposure chamber 101 to the selectedflow rate in the range of approximately 10 to 200 cm³/min. However, theforegoing range of flow rates is included by way of example only, and itshould not be construed to limit the present invention in any way.

Referring to FIGS. 2C and 2D, one embodiment of the method 200 mayfurther include a step 208 of monitoring a degree of passivation of theNLO crystal 104. The degree of passivation may be correlated to anamount or change in amount of OH bonds of the NLO crystal 104 becausethe amount of OH bonds generally increases as the NLO crystal 104 ispassivated as a result of having hydrogen atoms attach to danglingoxygen bonds of the NLO crystal 104. Accordingly, the degree ofpassivation may be monitored by analyzing one or more absorption bandsof the NLO crystal 104, wherein the absorption band is affected by achange in the number of OH bonds of the NLO crystal 104. The absorptionband may be analyzed by using any method known to the art for detectinga level at which the NLO crystal 104 absorbs illumination having one ormore wavelengths. In one embodiment, the degree of passivation may bemonitored utilizing Fourier Transform Infrared Spectroscopy (FTIR). Forexample, utilizing Fourier Transform Infrared Spectroscopy (FTIR), thedegree of passivation of the NLO crystal 104 may be monitored throughthe observation of at least one absorption band in the Infrared (IR)spectrum of the NLO crystal 104. An FTIR process for monitoring thedegree of passivation of the NLO crystal 104 may include one or more ofthe following steps: (i) transmitting illumination having one or morewavelengths through the NLO crystal 104; (ii) detecting illuminationtransmitted through the NLO crystal 104; and (iii) determining an amountof illumination absorbed by the NLO crystal 104 at one or morewavelengths utilizing information about illumination transmitted throughthe NLO crystal 104; and (iv) determining the degree of passivation ofthe NLO crystal 104 utilizing a correlation between illuminationabsorbed by the NLO crystal 104 at one or more wavelengths and theamount or change in amount of OH bonds of the NLO crystal 104.

In a further embodiment of the method 200, the NLO crystal 104 may beexposed to passivating gas in step 204 until the NLO crystal 104 issufficiently passivated. The step 208 of monitoring the degree ofpassivation of the NLO crystal 104 may be utilized to determine whetheror not the NLO crystal 104 has been sufficiently passivated. Forexample, the degree of passivation of the NLO crystal 104 may bedetermined by observing one or more absorption bands of the NLO crystal104 appearing or changing intensity at one or more wavelengths of the IRspectrum in the range of approximately 3200 to 4000 cm⁻¹, wherein theamplitude or intensity of the absorption band appearing or changingintensity at the wavelength correlates to the amount or change in amountof OH bonds of the NLO crystal 104. For instance, FTIR may be used tomonitor the absorption of —OH bonds (including H₂O) near 3580 cm-1 inthe infra-red spectrum. For example, FTIR monitoring may be performedin-situ, wherein a crystal is monitored with FTIR while it is undergoingpassivation. Step 208 may further determine whether or not the NLOcrystal 104 has been sufficiently passivated by monitoring the relativechange in the integrated peak intensity of one or more selected peaks inthe FTIR absorption spectra. For instance, step 208 may determinesufficient passivation when a 5% reduction in an —OH absorption peak isobserved.

The foregoing range of absorption band wavelengths and the percentagechange for sufficient passivation are included by way of example onlyand it is contemplated that one or more absorption bands may appear atother wavelengths in the IR, visible, and/or UV spectra; accordingly,the foregoing range of wavelengths is not intended to limit the presentinvention in any way.

The foregoing steps are neither sequential nor mandatory and may occurin any order or concurrent with one another. For example, it iscontemplated that in one embodiment of the method 200, the NLO crystal104 may be exposed to passivating gas as provided for in step 204; andconcurrently, the degree of passivation of the NLO crystal 104 may bemonitored utilizing FTIR as provided for in step 208. In some instancesit may be advantageous to combine some or all of the steps and toarrange the steps in a sequence that departs from the order in which thesteps have been discussed herein. The discussion herein is explanatoryonly and is not intended to limit the method or methods disclosed hereinto any particular sequence, order, or combination of steps.

FIGS. 3A through 3D illustrate a method 300 for passivating andannealing the NLO crystal 104. Referring to FIG. 3A, the method 300 mayinclude one or more of the following steps: (i) step 302, performing anannealing process on the NLO crystal 104 to reduce the water or OHcontent of the NLO crystal 104; and (ii) step 304, exposing the NLOcrystal 104 to passivating gas having a selected concentration ofhydrogen that is a user selected hydrogen concentration or a hydrogenconcentration determined utilizing one or more attributes of the NLOcrystal 104.

In step 302, the NLO crystal 104 may undergo an annealing process in adry atmosphere (e.g. clean dry air or dry inert gas) to remove at leasta portion of water or OH molecules from the NLO crystal 104. Annealingprocesses are known to the art and may include one or more of thefollowing steps: (i) increasing or decreasing the temperature of the NLOcrystal 104 to a selected temperature such as a sufficiently high valuefor removing water molecules from the NLO crystal 104 without melting ordamaging the NLO crystal 104; (ii) maintaining the temperature of theNLO crystal 104 at or near the selected temperature for a selectedperiod of time such as a sufficient period of time to decrease watercontent of the NLO crystal 104 to a selected level; and (iii) increasingor decreasing the temperature of the NLO crystal 104 to a selected finaltemperature such as ambient or room temperature when water content ofthe NLO crystal 104 has been reduced to the selected level. The selectedlevel of water content may be a user selected level, a water contentlevel determined utilizing one or more attributes of the NLO crystal104, or any water content level correlating to desired optical/physicalperformance or increased crystal lifetime.

In one embodiment, the annealing process of step 302 may further includea step of increasing or decreasing the temperature of the NLO crystal104 to the selected temperature over a selected time interval. Forexample, the NLO crystal 104 may be heated to the selected temperatureof approximately 150° C. gradually over the course of the selected timeperiod of approximately 2 hours. The temperature of the NLO crystal 104may be increased, decreased, or maintained by any known heating orcooling device. For instance, the substrate 102 may be equipped with aheating or cooling device suitable for heating or cooling the NLOcrystal 104. In another instance, the chamber 101 may be configured asan oven or a refrigerator. The heating or cooling device may be furtherconfigured to maintain the temperature of the NLO crystal 104 at or nearthe selected temperature for a selected period of time such as a userselected time period or a time period determined utilizing one or moreattributes of the NLO crystal 104. For example, the temperature of theNLO crystal 104 may be maintained at or near 150° C. for approximately10 hours. Alternatively, the temperature of the NLO crystal 104 may bemaintained at or near the selected temperature until the water or OHcontent of the NLO crystal 104 is sufficiently reduced. The foregoingtemperatures, time periods, and time intervals are included by way ofexample only, and it is contemplated that these parameters may besignificantly altered without departing from the essence of thisdisclosure. Accordingly, nothing herein should be construed to limit thepresent invention in any way.

In a further embodiment, the annealing process of step 302 may berepeated to further reduce the water content of the NLO crystal 104. Theannealing process may be repeated utilizing the same or differentparameters if necessary, such as one or more different temperatures ordifferent time periods or intervals. For example, the NLO crystal 104may be heated to approximately 200° C. over the course of approximately1 hour. Similarly, the temperature of the NLO crystal 104 may bemaintained at or near 200° C. for approximately 100 hours or until thewater or OH content of the NLO crystal 104 is sufficiently reduced. Theforegoing temperatures, time periods, and time intervals are included byway of example only, and it is contemplated that these parameters may besignificantly altered without departing from the essence of thisdisclosure. Accordingly, nothing herein should be construed to limit thepresent invention in any way.

The annealing process of step 302 may further include the step ofgradually increasing or decreasing the temperature of the NLO crystal104 to the selected final temperature (e.g. ambient or room temperature)over a selected time interval. For example, the NLO crystal 104 may begradually cooled or allowed to cool to ambient or room temperature overthe course of approximately 3 hours or any other acceptable timeinterval. In one embodiment, the NLO crystal 104 may be cooled by havingheat gradually removed so that the temperature of the NLO crystal 104gradually decreases to ambient temperature over the selected timeinterval. In another embodiment, the NLO crystal 104 may be cooledutilizing a cooling device to decrease the temperature of the NLOcrystal 104 to the selected final temperature. The selected timeinterval may be any user selected time interval or a time intervaldetermined utilizing one or more attributes of the NLO crystal 104.Accordingly, any time interval included herein is included by way ofexample only and is not intended to limit the present invention in anyway.

Referring to FIGS. 3B and 3D, the annealing process of step 302 mayfurther include a step 310 of monitoring the water or OH content of theNLO crystal by analyzing one or more absorption bands of the NLO crystal104, wherein the absorption band is affected by a change in the numberof OH bonds of the NLO crystal 104. The absorption band may be analyzedby using any method known to the art for detecting a level at which theNLO crystal 104 absorbs illumination having one or more wavelengths. Forexample, utilizing FTIR, the water or OH content of the NLO crystal 104may be monitored by observing at least one absorption band in theInfrared (IR) spectrum of the NLO crystal 104. An FTIR process formonitoring the water or OH content of the NLO crystal 104 may includeone or more of the following steps: (i) transmitting illumination havingone or more wavelengths through the NLO crystal 104; (ii) detectingillumination transmitted through the NLO crystal 104; and (iii)determining an amount of illumination absorbed by the NLO crystal 104 atone or more wavelengths utilizing information about illuminationtransmitted through the NLO crystal 104; and (iv) determining the wateror OH content or change in water or OH content of the NLO crystal 104utilizing a correlation between illumination absorbed by the NLO crystal104 at one or more wavelengths and the amount or change in amount of OHbonds of the NLO crystal 104.

In a further embodiment, the annealing process of step 302 may furtherinclude a step 312 of performing one or more steps of the annealingprocess until a determination is made utilizing the monitoring processof step 310 that the water or OH content of the NLO crystal has beensufficiently reduced. For example, the water or OH content of the NLOcrystal 104 may be determined by observing one or more absorption bandsof the NLO crystal 104 appearing at one or more wavelengths of the IRspectrum in the range of approximately 3200 to 4000 cm⁻¹, wherein theamplitude or intensity of the absorption band appearing at thewavelength correlates to the amount or change in amount of OH bonds ofthe NLO crystal 104. The foregoing range of absorption band wavelengthsis included by way of example only and it is contemplated that one ormore absorption bands may appear at other wavelengths in the IRspectrum; accordingly, the foregoing range of wavelengths is notintended to limit the present invention in any way.

The foregoing steps of the annealing process of step 302 are neithersequential nor mandatory. The steps may occur in any order or concurrentwith one another. For example, it is contemplated that the NLO crystal104 may be maintained at the selected temperature; concurrently, thewater or OH content of the NLO crystal 104 may be monitored utilizingFTIR as provided for by step 310. It is further contemplated that thetemperature of the NLO crystal 104 may be maintained at the selectedtemperature until the water or OH content of the NLO crystal 104 hasbeen sufficiently reduced as provided for by step 312. In some instancesit may be advantageous to combine some or all of the steps and toarrange the steps in a sequence that departs from the order in which thesteps have been discussed herein. The discussion herein is explanatoryonly and is not intended to limit the method or methods disclosed hereinto any particular sequence, order, or combination of steps.

After the NLO crystal 104 has been annealed to reduce the water or OHcontent of the NLO crystal 104, it may be advantageous to passivate theNLO crystal 104 with hydrogen to cure crystal defects caused by one ormore dangling or broken bonds, some of which may have resulted from theannealing process of step 302. Accordingly, in step 304 of the method300 the NLO crystal 104 may be exposed to passivating gas within acontainer such as the exposure chamber 101 of the system 100. Thepassivating gas may be a gas mixture having a selected concentration ofhydrogen. The hydrogen concentration may be a user selectedconcentration, a concentration determined utilizing one or moreattributes of the NLO crystal 104, or any acceptable concentration forcuring crystal defects of the NLO crystal 104 by attaching hydrogenatoms from the passivating gas to broken or dangling bonds of the NLOcrystal 104. For example, in one embodiment the selected hydrogenconcentration of the passivating gas may be a hydrogen concentration inthe range of approximately 5 to 10% of the passivating gas mixture.However, the foregoing hydrogen concentration is only included by way ofexample, and it is not intended to limit the present invention in anyway. In some embodiments, step 304 may further include one or more stepsor elements from the method 200 of passivating the NLO crystal 104,previously discussed.

Referring to FIGS. 3C and 3D, the passivating process of step 304 mayfurther include a step 320 of monitoring the degree of passivation ofthe NLO crystal 104. The degree of passivation may be monitored byanalyzing one or more absorption bands of the NLO crystal 104, whereinthe absorption band is affected by a change in the number of OH bonds ofthe NLO crystal 104. The absorption band may be analyzed by using anymethod known to the art for detecting a level at which the NLO crystal104 absorbs illumination having one or more wavelengths. For example,utilizing FTIR, the degree of passivation of the NLO crystal 104 may bemonitored by observing at least one absorption band in the Infrared (IR)spectrum of the NLO crystal 104. An FTIR process for monitoring thedegree of passivation of the NLO crystal 104 may include one or more ofthe following steps: (i) transmitting illumination having one or morewavelengths through the NLO crystal 104; (ii) detecting illuminationtransmitted through the NLO crystal 104; and (iii) determining an amountof illumination absorbed by the NLO crystal 104 at one or morewavelengths utilizing information about illumination transmitted throughthe NLO crystal 104; and (iv) determining the degree of passivation ofthe NLO crystal 104 utilizing a correlation between illuminationabsorbed by the NLO crystal 104 at one or more wavelengths and theamount or change in amount of OH bonds of the NLO crystal 104.

In a further embodiment, step 304 may further include a step 322 ofexposing the NLO crystal 104 to passivating gas until the NLO crystal104 is sufficiently passivated. The step 320 of monitoring the degree ofpassivation of the NLO crystal 104 may be utilized to determine whetheror not the NLO crystal 104 has been sufficiently passivated. Forexample, the degree of passivation of the NLO crystal 104 may bedetermined by observing one or more absorption bands of the NLO crystal104 appearing or changing intensity at one or more wavelengths of the IRspectrum in the range of approximately 3200 to 4000 cm⁻¹, wherein theamplitude or intensity of the absorption band appearing or changingintensity at the wavelength correlates to the amount or change in amountof OH bonds of the NLO crystal 104. The foregoing range of absorptionband wavelengths is included by way of example only and it iscontemplated that one or more absorption bands may appear at otherwavelengths in the IR spectrum; accordingly, the foregoing range ofwavelengths is not intended to limit the present invention in any way.

The foregoing steps are neither sequential nor mandatory and may occurin any order or concurrent with one another. For example, it iscontemplated that in one embodiment of step 304, the NLO crystal 104 maybe exposed to passivating gas having the selected concentration ofhydrogen; and concurrently, the degree of passivation of the NLO crystal104 may be monitored utilizing FTIR as provided for in step 320. It isfurther contemplated that the NLO crystal may be exposed to passivatinggas until the NLO crystal 104 has been sufficiently passivated asprovided for in step 322, wherein the monitoring technique of step 320may be utilized to determine whether or not the NLO crystal 104 has beensufficiently passivated. In some instances it may be advantageous tocombine some or all of the steps and to arrange the steps in a sequencethat departs from the order in which the steps have been discussedherein. The discussion herein is explanatory only and is not intended tolimit the method or methods disclosed herein to any particular sequence,order, or combination of steps.

It may be advantageous to incorporate the NLO crystal 104, having beensufficiently annealed and passivated, into a laser system for betterphysical/optical performance or greater crystal lifetime than could beachieved utilizing an unmodified NLO crystal 104. The laser systemconfiguration of the present disclosure may include, but is not limitedto, configurations such as mode-locked, CW, Q-switched, and any otherlaser or laser system including one or more nonlinear crystals. Thedescription herein is further intended to include a broad range ofpossible laser spectra, including but not limited to electromagneticspectra such as Deep Ultraviolet (DUV), Ultraviolet (UV), Infrared,visible, and the like. As used herein, the terms “laser system” and“laser” may be used interchangeably to describe a configuration of oneor more lasers.

FIG. 4 illustrates a laser system 400 equipped with a passivated and/orannealed NLO crystal 104. The laser system 400 of the present inventionmay include, but is not limited to, a light source 402, a first set ofbeam shaping optics 404, the passivated/annealed crystal 104 asdescribed previously herein, a housing unit 406, a set of harmonicseparation elements 408, and a second set of beam shaping optics 410.

In one aspect, the output of a light source 402 may be focused to anelliptical cross-section Gaussian beam waist in or proximate to apassivated/annealed NLO crystal 104 using beam shaping optics 404. Asused herein, the term “proximate to” is preferably less than half of theRayleigh range from the center of crystal 104. In one embodiment, theaspect ratio between the Gaussian widths of the principle axes of theellipse may fall between about 2:1 and about 6:1. In other embodimentsthe ratio between the principle axes of the ellipse may be between about2:1 and about 10:1. In one embodiment, the wider Gaussian width issubstantially aligned with the walk-off direction of the NLO crystal 104(e.g. to within about 10° of alignment).

In another aspect, the housing unit 406 may protect the NLO crystal 104from ambient atmospheric conditions and other impurities, therebyfacilitating maintenance of its passivated/annealed condition. Note thata crystal exposed to atmospheric water and other impurities over timewill begin to deteriorate and may revert back to an unpassivated orun-annealed state. Crystal housing units are described generally in U.S.patent application Ser. No. 12/154,337, entitled “Enclosure ForControlling The Environment of Optical Crystals”, filed May 6, 2008,which is incorporated herein by reference in the entirety. In someembodiments, housing unit 406 may include a large structure suitable forhousing crystal 104 and other components of the laser system 400. Inother embodiments, housing 406 may be large enough to house allcomponents of the laser system 400. Note that the larger the housing,the more precautions needed for maintenance and repair of the lasersystem (to protect crystal 104 from degradation and maintain itspassivated/annealed condition). As such, in further aspects, the housingunit 406 may consist of a small housing structure suitable for enclosingprimarily only the NLO crystal 104.

Beam shaping optics 404 may include anamorphic optics, which may changethe cross section of output from light source 402. Anamorphic optics mayinclude, for example, at least one of a prism, a cylindrical curvatureelement, a radially-symmetric curvature element, and a diffractiveelement. In one embodiment, light source 402 may include a laserproducing a frequency in the visible range (e.g. 532 nm) to be doubledinside crystal 104. In other embodiments, light source 104 may include alaser source producing two or more frequencies to be combined insidecrystal 402 to generate a sum or difference frequency. Frequencyconversion and associated optics and hardware are described Dribinski etal. in U.S. patent application Ser. No. 13/412,564, filed on Mar. 6,2012, which is incorporated herein by reference in the entirety.

FIG. 5 illustrates an inspection system 500 configured for measuring oranalyzing defects of one or more samples 510, such as a photomask (i.e.,a reticle), wafer, or any other sample that may be analyzed utilizing anoptical inspection system. The inspection system 500 may include a lasersystem 400 as described above. The laser system 400 may include one ormore of the passivated/annealed NLO crystals 104 described throughoutthe present disclosure. In one embodiment, the NLO crystal 104 of thelaser system 400 may be sufficiently annealed to reduce the watercontent of the NLO crystal 104 to a selected water content level.

In a further embodiment, the NLO crystal 104 of the laser system 400 maybe sufficiently passivated to cure crystal defects caused by dangling orbroken bonds, such as dangling oxygen bonds. Dangling or broken bonds ofthe NLO crystal 104 may be cured through passivation by bonding hydrogenatoms to the broken or dangling bonds of the NLO crystal 104. In somecases, a portion of dangling or broken bonds may be products of theannealing process performed on the NLO crystal 104. The NLO crystal 104may be passivated to a selected degree of passivation that is acceptablefor achieving desired physical/optical performance, improved LIDresistance, improved output beam quality, improved output stability,increased crystal lifetime, or higher operating power.

The NLO crystal 104 of the laser system 400 may have at least oneabsorption band in the IR spectrum of the NLO crystal 104 correlated tothe presence, absence, or amount of OH bonds of the NLO crystal 104. Theabsorption band of the NLO crystal 104 may be measured utilizing FTIR todetermine the degree of passivation or the water content level of theNLO crystal 104. A specified amplitude or intensity of the absorptionband of the NLO crystal 104 may correspond to the sufficient annealinglevel or the sufficient passivating level of the NLO crystal 104. Thespecified amplitude or intensity of the absorption band may be a userselected value, or a value determined utilizing one or more attributesof the NLO crystal 104. Accordingly, the absorption band of NLO crystal104 of the laser system 400 may have an amplitude or intensity at ornear the specified amplitude or intensity. The laser system 400 mayfurther include at least one electromagnetic source, such as a diodepumped solid state (DPSS) source or a fiber IR source, configured toprovide illumination to the NLO crystal 104. At least a portion of theillumination provided by the electromagnetic source may be directly orindirectly transmitted through the NLO crystal 104 in a frequencyconversion process of the crystal 104.

The inspection system 500 may further include a sample stage 512configured to hold the sample 510 during the inspection process. Thesample stage 512 may be configured to hold the sample 510 in a locationwhere the sample 510 may receive at least a portion of illuminationtransmitted from the laser system 400. The sample stage 512 may befurther configured to actuate the sample 510 to a user selectedlocation. The sample stage 512 may further be communicatively coupled toone or more computing systems and configured to actuate the sample 510to the user selected location or to a location determined by thecomputing system, wherein the sample 510 may receive at least a portionof illumination transmitted from the laser system 400.

The inspection system 500 may further include a detector 504 configuredto directly or indirectly receive at least a portion of illuminationreflected from a surface of the sample 510. The detector 504 may includeany suitable detector known to the art, such as a charged coupled device(CCD) or a time-delay-and-integration (TDI) CCD based detector. Theinspection system 500 may further include one or more computing systems514 communicatively coupled to the detector 504. The computing system514 may be configured to receive information regarding characteristicsof illumination reflected from the surface of the sample 510 from thedetector 504. The computing system 514 may be further configured toexecute an inspection algorithm from program instructions 418 on acarrier medium 416. The inspection algorithm 420 may be any inspectionalgorithm known to the art for measuring one or more defects of thesample 510 utilizing information regarding characteristics ofillumination reflected from the surface of the sample 510. Accordingly,the computing system 514 may utilize information regarding illuminationreflected from the surface of the sample 510 to make measurements, suchas presence, absence, quantity, and/or type of defects of the sample510.

The inspection system 500 may include one or more illumination opticalelements 503 (e.g. retarders, quarter wave plates, focus optics, phasemodulators, polarizers, mirrors, beam splitters, reflectors,converging/diverging lenses, prisms, etc.). The illumination opticalelements 503 may be configured to directly or indirectly receiveillumination emanating from the laser system 400. The illuminationoptical elements 503 may be further configured to transmit and/or directat least a portion of illumination directly or indirectly received fromthe laser system 400 along an illumination path of the inspection system500 to the surface of the sample 510. The illumination path may be anypath along which illumination can travel from the laser system 400 tothe surface of the sample 510, such as a direct line of sight betweenthe laser system 400 and the surface of the sample 510. In someembodiments, the illumination path may be a path delineated by aconfiguration of one or more optical elements including, but not limitedto, the illumination optical elements or any other optical elementsdisclosed herein.

In one embodiment, the illumination path of the inspection system 400may include a beam splitter 508 configured to transmit at least aportion of illumination received directly or indirectly from the lasersystem 400 to the surface of the sample 510 or to a further component ofthe illumination path. The beam splitter 508 may be any optical devicecapable of splitting a beam of illumination into two or more beams ofillumination. The illumination path may further include inspectionoptical elements 505 (e.g. retarders, quarter wave plates, focus optics,phase modulators, polarizers, mirrors, beam splitters, reflectors,converging/diverging lenses, prisms, etc.) configured to transmit atleast a portion of illumination received directly or indirectly from thelaser system 400 to the surface of the sample 510.

In one embodiment the inspection system 500 may include collectionoptical elements 505 (e.g. retarders, quarter wave plates, focus optics,phase modulators, polarizers, mirrors, beam splitters, reflectors,converging/diverging lenses, prisms, etc.) configured to directly orindirectly receive at least a portion of illumination reflected from thesurface of the sample 510. The collection optical elements 506 may befurther configured to transmit at least a portion of illuminationdirectly or indirectly received from the surface of the sample 510 alonga collection path of the inspection system 500 to the detector 504. Thecollection path may be any path along which illumination can travel fromthe surface of the sample 510 to the detector 504, such as a direct lineof sight between the surface of the sample 510 and the detector 504. Insome embodiments, the collection path may be a path delineated by aconfiguration of one or more optical elements including, but not limitedto, the collection optical elements or any other optical elementsdisclosed herein.

While the present disclosure describes the inspection system 400 in thecontext of generically inspecting one or more samples, it iscontemplated that the inventive aspects of the inspection system 500 maybe extended to wide array of inspection or metrology systems utilized inthe fabrication or analysis of semiconductors or semiconductorcomponents. The inspection system 500 may be configured for one or moremodes of operation known to the art. For example, the inspection system500 may be configured for bright-field inspection, dark-fieldinspection, or any other mode or configuration now or hereafter known tothe art. The inspection system 500 may be further configured for one ormore inspection capabilities known to the art. For example, theinspection system 500 may be configured for inspecting one or morephotomasks, patterned wafers, unpatterned wafers, or any otherinspection capability now or hereafter known to the art.

It should be recognized that the various steps described throughout thepresent disclosure may be carried out by a single computing system or,alternatively, a multiple computing system. Moreover, differentsubsystems of the system may include a computing system suitable forcarrying out at least a portion of the steps described above. Therefore,the above description should not be interpreted as a limitation on thepresent invention but merely an illustration. Further, the one or morecomputing systems may be configured to perform any other step(s) of anyof the method embodiments described herein.

The computing system may include, but is not limited to, a personalcomputing system, mainframe computing system, workstation, imagecomputer, parallel processor, or any other device known in the art. Ingeneral, the term “computing system” may be broadly defined to encompassany device having one or more processors, which execute instructionsfrom a memory medium.

Program instructions implementing methods such as those described hereinmay be transmitted over or stored on carrier medium. The carrier mediummay be a transmission medium such as a wire, cable, or wirelesstransmission link. The carrier medium may also include a storage mediumsuch as a read-only memory, a random access memory, a magnetic oroptical disk, or a magnetic tape.

All of the methods described herein may include storing results of oneor more steps of the method embodiments in a storage medium. The resultsmay include any of the results described herein and may be stored in anymanner known in the art. The storage medium may include any storagemedium described herein or any other suitable storage medium known inthe art. After the results have been stored, the results can be accessedin the storage medium and used by any of the method or systemembodiments described herein, formatted for display to a user, used byanother software module, method, or system, etc. Furthermore, theresults may be stored “permanently,” “semi-permanently,” temporarily, orfor some period of time. For example, the storage medium may be randomaccess memory (RAM), and the results may not necessarily persistindefinitely in the storage medium.

It is further contemplated that each of the embodiments of the methoddescribed above may include any other step(s) of any other method(s)described herein. In addition, each of the embodiments of the methoddescribed above may be performed by any of the systems described herein.

Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems and/or other technologiesdescribed herein can be effected (e.g., hardware, software, and/orfirmware), and that the preferred vehicle will vary with the context inwhich the processes and/or systems and/or other technologies aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a mainly hardwareand/or firmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a mainly software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes and/or devices and/or other technologies describedherein may be effected, none of which is inherently superior to theother in that any vehicle to be utilized is a choice dependent upon thecontext in which the vehicle will be deployed and the specific concerns(e.g., speed, flexibility, or predictability) of the implementer, any ofwhich may vary. Those skilled in the art will recognize that opticalaspects of implementations will typically employ optically-orientedhardware, software, and or firmware.

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use engineering practices to integrate such describeddevices and/or processes into data processing systems. That is, at leasta portion of the devices and/or processes described herein can beintegrated into a data processing system via a reasonable amount ofexperimentation. Those having skill in the art will recognize that atypical data processing system generally includes one or more of asystem unit housing, a video display device, a memory such as volatileand non-volatile memory, processors such as microprocessors and digitalsignal processors, computational entities such as operating systems,drivers, graphical user interfaces, and applications programs, one ormore interaction devices, such as a touch pad or screen, and/or controlsystems including feedback loops and control motors (e.g., feedback forsensing position and/or velocity; control motors for moving and/oradjusting components and/or quantities). A typical data processingsystem may be implemented utilizing any suitable commercially availablecomponents, such as those typically found in datacomputing/communication and/or network computing/communication systems.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “connected”, or “coupled”, toeach other to achieve the desired functionality, and any two componentscapable of being so associated can also be viewed as being “couplable”,to each other to achieve the desired functionality. Specific examples ofcouplable include but are not limited to physically mateable and/orphysically interacting components and/or wirelessly interactable and/orwirelessly interacting components and/or logically interacting and/orlogically interactable components.

While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims are toencompass within their scope all such changes and modifications as arewithin the true spirit and scope of the subject matter described herein.

Furthermore, it is to be understood that the invention is defined by theappended claims.

Although particular embodiments of this invention have been illustrated,it is apparent that various modifications and embodiments of theinvention may be made by those skilled in the art without departing fromthe scope and spirit of the foregoing disclosure. Accordingly, the scopeof the invention should be limited only by the claims appended hereto.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

What is claimed:
 1. A method comprising: providing a nonlinear optical(NLO) crystal; maintaining a temperature of the NLO crystal within aselected temperature range below a melting temperature of the NLOcrystal; and exposing the NLO crystal to passivating gas having aconcentration of at least one of hydrogen, deuterium, ahydrogen-containing compound or a deuterium-containing compound at ornear a selected concentration to achieve a selected passivation levelwithin the NLO crystal.
 2. The system of claim 1, wherein thepassivating gas comprises: a mixture of hydrogen, deuterium and an inertgas.
 3. The method of claim 1, wherein the passivating gas comprises: atleast one inert gas mixed with at least one of hydrogen, deuterium, ahydrogen-containing compound or a deuterium-containing compound.
 4. Themethod of claim 1, wherein the maintaining a temperature of the NLOcrystal within a selected temperature range comprises: maintaining atemperature of the NLO crystal between room temperature and a meltingpoint of the NLO crystal.
 5. The method of claim 4, wherein themaintaining a temperature of the NLO crystal within a selectedtemperature range comprises: maintaining a temperature of the NLOcrystal temperature between 300° C. and 350° C.
 6. The system of claim1, wherein the NLO crystal comprises: at least one of Beta-Barium Borate(BBO), Lithium Triborate (LBO), Lithium Tetraborate (LTB), CesiumLithium Borate (CLBO), or Cesium Borate (CBO).
 7. The system of claim 1,further comprising: containing the NLO crystal within a crystal housingunit.
 8. The system of claim 1, further comprising: transmitting a beamof light from a light source through the NLO crystal.
 9. The system ofclaim 8, further comprising: transmitting the beam of light from thelight source through the NLO crystal to generate deep ultraviolet lightwith the NLO crystal.
 10. The system of claim 8, further comprising:shaping the beam of light from the light source with one or more beamshaping optics.
 11. A method comprising: providing an NLO crystal;performing an annealing process on the NLO crystal; and exposing the NLOcrystal to passivating gas having a concentration of at least one ofhydrogen, deuterium, a hydrogen-containing compound and adeuterium-containing compound at or near a selected concentration toachieve a selected passivation level within the NLO crystal.
 12. Thesystem of claim 11, wherein the passivating gas comprises: a mixture ofhydrogen, deuterium and an inert gas.
 13. The method of claim 11,wherein the passivating gas comprises: at least one inert gas mixed withat least one of hydrogen, deuterium, a hydrogen-containing compound or adeuterium-containing compound.
 14. The method of claim 11, wherein theperforming an annealing process on the NLO crystal to reduce water or OHcontent of the NLO crystal comprises: performing an annealing process onthe NLO crystal between room temperature and a melting point of the NLOcrystal.
 15. The method of claim 14, wherein the performing an annealingprocess on the NLO crystal to reduce water or OH content of the NLOcrystal comprises: performing an annealing process on the NLO crystaltemperature between 300° C. and 350° C.
 16. The system of claim 11,wherein the NLO crystal comprises: at least one of Beta-Barium Borate(BBO), Lithium Triborate (LBO), Lithium Tetraborate (LTB), CesiumLithium Borate (CLBO), or Cesium Borate (CBO).
 17. The system of claim11, further comprising: containing the NLO crystal within a crystalhousing unit.
 18. The system of claim 11, further comprising:transmitting a beam of light from a light source through the NLOcrystal.
 19. The system of claim 18, further comprising: transmittingthe beam of light from the light source through the NLO crystal togenerate deep ultraviolet light with the NLO crystal.
 20. The system ofclaim 18, further comprising: shaping the beam of light from the lightsource with one or more beam shaping optics.